The term of protection shall be determined by the Office for each species. It may not be less than 20 years for agricultural crops and not less than 25 years for trees and vines. The term of protection shall begin with the issue of the certificate. CHAPTER IV TRANSFER AND LOSS OF RIGHTS Article 20 The rights deriving from an application for a certificate or from a certificate may be transferred in whole or in part. They may be the subject, in whole or in part, of the grant of an exclusive or nonexclusive exploitation license. The rights afforded by an application for a certificate or a certificate may be invoked with respect to a licensee who fails to comply with the limits set out in the license in accordance with the preceding paragraph. Subject to the case referred to in Article 61 below, transfer of the rights referred to in the first paragraph shall not affect rights acquired by third parties prior to the date of the transfer. The acts comprising a transfer or a license as referred to in the first two paragraphs shall be set out in writing, on pain of nullity. Article 21 Any public or private law person may, on expiry of three years after the issue of a certificate or of four years after the filing date of an application, obtain a compulsory license under that certificate, subject to the conditions set out in Articles 22 to 24 below, if at the time of the request, and save for legitimate reasons, the holder of the certificate or his successor in title: (a) has not begun to exploit the subject matter of the certificate on the territory of Morocco nor has made effective and serious preparation to exploit it or (b) has not marketed the product which is the subject matter of the certificate in a quantity sufficient to satisfy the needs of the domestic market or (c) if exploitation or marketing of the variety in Morocco has been abandoned for more than three years. Article 22 The application for a compulsory license shall be presented to the competent court. It shall be accompanied by proof that the applicant was unable to obtain a license from the